At what distance should an object be placed from a convex lens of focal langth `15 cm` to obtain an image three times the size of the object ?
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Here, `u = ?, f = 15cm, m = +-3 = (v)/(u)` `:. v = +-3 3u` From `(1)/(v) - (1)/(u) = (1)/(f)` `(1)/(+- 3u) - (1)/(u) = (1)/(15)` `:. u = 20 cm`, when `m` is negative, `u = 10cm`, when `m` is positive, i.e., for virtual image, `u = 10 cm` and for reqal image, `u = 20cm`
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